Corrosion Resistant Conversion Coatings

Abstract

A conversion coating composition for coating a metal substrate is provided which imparts corrosion resistance to the underlying metal substrate. The conversion coating composition comprises an aqueous carrier and first and second rare earth element salts. A complete coating system employing the conversion coating composition is also provided as well as methods for conversion coating a metal substrate with the rare earth element conversion coating compositions of the present invention.

1 . A composition for coating a metal substrate comprising an aqueous carrier and first and second salts, each salt comprising an anion and a cation, the anion of the first and second salts being different, and the cation of the first and second salts being the same or different, wherein each cation, individually, is a rare earth element, and the first and second salts in combination are present in the composition in an amount effective to form a corrosion resistant coating on the metal substrate. 
   
   
       2 . A composition according to  claim 1  wherein at least one salt is capable of reaction at or near the surface of a metal substrate to form a coating on the metal substrate. 
   
   
       3 . A composition according to  claim 1  wherein the first and second salts in combination are present in the composition in at least about 0.04 weight percent. 
   
   
       4 . A composition according to  claim 3  wherein the first and second salts in combination are present in the composition in at least about 1.5 weight percent. 
   
   
       5 . A composition according to  claim 4  wherein the first and second salts in combination are present in the composition in about 8 weight percent. 
   
   
       6 . A composition according to  claim 1  wherein at least one cation is a Cerium (III) cation. 
   
   
       7 . A composition according to  claim 6  wherein at least two cations are Cerium (III) cations. 
   
   
       8 . A composition according to  claim 6  wherein at least one of the first and second salts is a Cerium (III) halide or Cerium (III) nitrate. 
   
   
       9 . A composition according to  claim 1  further comprising an oxidizing agent. 
   
   
       10 . A composition according to  claim 9  wherein the oxidizing agent is hydrogen peroxide. 
   
   
       11 . A composition according to  claim 1  further comprising a self-cleaning additive. 
   
   
       12 . A composition according to  claim 11  wherein the self-cleaning additive is a surfactant or a detergent. 
   
   
       13 . A composition according to  claim 1  further comprising an oxidizing agent, and wherein each cation of the first and second salts is a different rare earth element cation. 
   
   
       14 . A composition for coating a metal substrate comprising water, an oxidizing agent, and first and second salts, wherein the first and second salts in combination are present in the composition in at least about 0.04 weight percent, and wherein each salt comprises at least two different anions of the same rare earth element cation. 
   
   
       15 . A composition according to  claim 14  wherein the first and second salts in combination are present in the composition in an amount effective to form a corrosion resistant coating on the metal substrate. 
   
   
       16 . A composition for coating a metal substrate comprising a plating bath having the dissolution products from one or more rare earth element oxides and an acid generating compound, wherein at least one dissolution product from a rare earth element oxide is present in the plating bath in an amount effective to form a corrosion resistant coating on the metal substrate. 
   
   
       17 . A composition according to  claim 16  wherein the acid generating compound is selected from the group consisting of gypsum, anhydrite, celestite, and barite, in hydrous and anhydrous forms, in naturally occurring mineral forms, and as precipitated salts. 
   
   
       18 . A composition according to  claim 16  further comprising an oxidant. 
   
   
       19 . A composition according to  claim 18  further comprising an additive. 
   
   
       20 . A metal substrate coating comprising:
 a) a conversion coating prepared from a composition having first and second salts, each salt comprising an anion and a cation, the anion of the first and second salts being different, and the cation of the first and second salts being the same or different, wherein each cation, individually, is a rare earth element, and the first and second salts in combination are present in the composition in an amount effective to form a corrosion resistant coating on the metal substrate; and   b) a primer coat.   
   
   
       21 . A metal substrate coating according to  claim 20  wherein the primer coat comprises a chromate based coating composition. 
   
   
       22 . A metal substrate coating according to  claim 20  wherein the primer coat comprises a chromate-free, —coating composition. 
   
   
       23 . A metal substrate coating according to  claim 20  further comprising a topcoat. 
   
   
       24 . A metal substrate coating according to  claim 23  wherein the topcoat is an advanced performance topcoat. 
   
   
       25 . A metal substrate coating comprising:
 a) a conversion coating prepared from a composition having first and second salts, each salt comprising an anion and a cation, the anion of the first and second salts being different, and the cation of the first and second salts being the same or different, wherein each cation, individually, is a rare earth element, and the first and second salts in combination are present in the composition in an amount effective to form a corrosion resistant coating on the metal substrate; and   b) a self-priming top-coat or an enhanced self-priming topcoat.   
   
   
       26 . A process for coating a metal substrate comprising:
 a) providing a metal substrate; and   b) coating the metal substrate with a composition according to  claim 1 .   
   
   
       27 . A process according to  claim 26  further comprising pre-treating the metal substrate prior to placing the coating on the metal substrate. 
   
   
       28 . The process according to  claim 26  wherein the pre-treating comprises pre-cleaning the metal substrate prior to placing the coating on the metal substrate. 
   
   
       29 . The process according to  claim 26  wherein the pre-treating comprises deoxidizing the metal substrate prior to placing the coating on the metal substrate. 
   
   
       30 . A process for coating a metal substrate according to  claim 26 , wherein the coating composition further comprises an oxidizing agent and the first and second salts in combination are present in the composition in at least about 0.04 weight percent. 
   
   
       31 . A method of coating a metal substrate comprising:
 a) providing a metal substrate having a surface;   b) contacting the metal substrate with a composition, the composition comprising an oxidizing agent and first and second salts, each salt comprising an anion and a cation, the anion of the first and second salts being different, and the cation of the first and second salts being the same or different, wherein each cation, individually, is a rare earth element, and the first and second salts, in combination, are present in the composition in at least about 0.04 weight percent; and   c) oxidizing at least one rare earth element cation, thereby resulting in the formation of a rare earth element oxide coating, a rare earth element hydroxide coating, or mixture thereof, at or near the surface of the metal substrate.